core: derive active permission profiles (#18277)

## Why

`Permissions` should not store a separate `PermissionProfile` that can
drift from the constrained `SandboxPolicy` and network settings. The
active profile needs to be derived from the same constrained values that
already honor `requirements.toml`.

## What changed

This adds derivation of the active `PermissionProfile` from the
constrained runtime permission settings and exposes that derived value
through config snapshots and thread state. The app-server can then
report the active profile without introducing a second source of truth.

## Verification

- `cargo test -p codex-core --test all permissions_messages --
--nocapture`
- `cargo test -p codex-core --test all request_permissions --
--nocapture`



























---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18277).
* #18288
* #18287
* #18286
* #18285
* #18284
* #18283
* #18282
* #18281
* #18280
* #18279
* #18278
* __->__ #18277

codex-rs/core/src/session/session.rs

@@ -90,6 +90,13 @@ impl SessionConfiguration {
         &self.codex_home
     }
 
+    pub(super) fn permission_profile(&self) -> PermissionProfile {
+        PermissionProfile::from_runtime_permissions(
+            &self.file_system_sandbox_policy,
+            self.network_sandbox_policy,
+        )
+    }
+
     pub(super) fn thread_config_snapshot(&self) -> ThreadConfigSnapshot {
         ThreadConfigSnapshot {
             model: self.collaboration_mode.model().to_string(),
@@ -98,6 +105,7 @@ impl SessionConfiguration {
             approval_policy: self.approval_policy.value(),
             approvals_reviewer: self.approvals_reviewer,
             sandbox_policy: self.sandbox_policy.get().clone(),
+            permission_profile: self.permission_profile(),
             cwd: self.cwd.clone(),
             ephemeral: self.original_config_do_not_use.ephemeral,
             reasoning_effort: self.collaboration_mode.reasoning_effort(),